Two empirically fitted parameters have been derived for 74 levels of theory. They allow fast and reliable pKa calculations using only the Gibbs energy difference between an acid and its conjugated base in aqueous solution (ΔGs(BA)). The parameters were obtained by least-squares fits of ΔGs(BA) vs experimental pKa values for phenols, carboxylic acids, and amines using training sets of 20 molecules for each chemical family. Test sets of 10 molecules per family-completely independent from the training set-were used to verify the reliability of the fitting parameters method. It was found that, except for MP2, deviations from experiments are lower than 0.5 pKa units. Moreover, mean unsigned errors lower than 0.35 pKa units were found for the 98.6%, 98.6%, and 94.6% of the tested levels of theory for phenols, carboxylic acids and amines, respectively. The parameters estimated here are expected to facilitate computationally based estimations of pKa values of species for which this magnitude is still unknown, with uncertainties similar to the experimental ones. However, the present study deals only with molecules of modest complexity, thus the reliability of the FP method for more complex systems remains to be tested.

Several aspects related to the antioxidant activity of ellagic acid were investigated using the density functional theory. It was found that this compound is unusually versatile for protecting against the toxic effects caused by oxidative stress. Ellagic acid, in aqueous solution at physiological pH, is able of deactivating a wide variety of free radicals, which is a desirable capability since in biological systems, these species are diverse. Under such conditions, the ellagic acid anion is proposed as the key species for its protective effects. It is predicted to be efficiently and continuously regenerated after scavenging two free radicals per cycle. This is an advantageous and unusual behavior that contributes to increase its antioxidant activity at low concentrations. In addition, the ellagic acid metabolites are also capable of efficiently scavenging a wide variety of free radicals. Accordingly, it is proposed that the ellagic acid efficiency for that purpose is not reduced after being metabolized. On the contrary, it provides continuous protection against oxidative stress through a free radical scavenging cascade. This is an uncommon and beneficial behavior, which makes ellagic acid particularly valuable to that purpose. After deprotonation, ellagic acid is also capable of chelating copper, in a concentration dependent way, decreasing the free radical production. In summary, ellagic acid is proposed to be an efficient multiple-function protector against oxidative stress.

The reactions of OH radicals with the neutral and anionic forms of Edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one, EDA) have been studied using Density Functional Theory. Different mechanisms and reaction sites have been considered. The overall rate constant was found to be diffusion-limited (1.35 × 10(10) M(-1) s(-1), in aqueous solution), and in excellent agreement with the experimental results. Therefore, the present work supports previous evidence that EDA is an excellent (•)OH scavenger. The anionic form of EDA is predicted to react 8.6 times faster than its neutral form. The preponderant mechanism was found to be different depending on the form of EDA reacting with the radical. For the anionic form, the single electron transfer mechanism was found to be the one contributing the most to the overall reactivity toward (•)OH (∼ 44%), closely followed by radical adduct formation (∼40%). For the neutral form, the latter was found to be the main mechanism, with contributions larger than 98%, regardless of the polarity of the environment. For the global reactivity of EDA toward OH radicals, at physiological pH, the main mechanism of reaction is proposed to be the sequential electron proton transfer. A detailed analysis of the UV-vis spectra is also provided. The excellent agreement with the available experimental data supports the reliability of the new information provided in the present work.

A systematic rational search for newly designed melatonin derivatives, was performed using a computer-assisted protocol. A total of 116 derivatives were generated by adding functional groups (i.e., -OH, -NH2, -SH and -COOH) to the melatonin structure. A selection score (SS) was built to sample the search space, simultaneously considering ADME (absorption, distribution, metabolism, excretion) properties, toxicity and manufacturability (i.e., synthetic accessibility). The search characterized the whole set of designed melatonin derivatives and allowed the selection of a reduced subset of 20 melatonin derivatives that are expected to be the most promising, regarding drug-like behavior. For this subset, several reactivity indices were estimated, as well as their pKa values. According to the gathered data, 5 melatonin derivatives have been identified as the most likely candidates to act as chemical antioxidant (directly scavenging free radicals, by electron transfer and/or H transfer). All of them are predicted to be better for that purpose than melatonin itself or trolox (water soluble vitamin E analog). The findings from this work are expected to motivate further investigations on these molecules, using both theoretical and experimental approaches.

Oxidative stress (OS) is a health-threatening process that is involved, at least partially, in the development of several diseases. Although antioxidants can be used as a chemical defense against OS, they might also exhibit pro-oxidant effects, depending on environmental conditions. In this work, such a dual behavior was investigated for phenolic compounds (PhCs) within the framework of the density functional theory and based on kinetic data. Multiple reaction mechanisms were considered in both cases. The presence of redox metals, the pH, and the possibility that PhCs might be transformed into benzoquinones were identified as key aspects in the antioxidant versus pro-oxidant effects of these compounds. The main virtues of PhCs as antioxidants are their radical trapping activity, their regeneration under physiological conditions, and their behavior as OH-inactivating ligands. The main risks of PhCs as pro-oxidants are predicted to be the role of phenolate ions in the reduction of metal ions, which can promote Fenton-like reactions, and the formation of benzoquinones that might cause protein arylation at cysteine sites. Although the benefits seem to overcome the hazards, to properly design chemical strategies against OS using PhCs, it is highly recommended to carefully explore their duality in this context.